1. Field of the Invention
The present invention relates to a worm reduction gear and a steering system including this worm reduction gear.
2. Description of Related Art
In an electric power steering system disclosed in Japanese Patent Application Publication No. 2003-334724 (JP 2003-334724 A), the rotation of a motor for generating steering assist force is transmitted to wheels through a worm and a worm wheel that meshes with this worm so as to change the steering angle. The worm wheel is formed by hobbing.
When the outside diameter of a hob used for hobbing a worm wheel is made closer to the outside diameter of a worm, the shape of each tooth space of the worm wheel after being bobbed more closely resembles the shape of a tooth of the worm. This configuration can theoretically reduce errors in torque transmission between the worm and the worm wheel (what is called “torque fluctuations”), However, in this case, there is no margin to allow assembling errors or machining errors of the worm wheel. Thus, when such errors occur, the positions where the tooth of the worm and the teeth of the worm wheel come into contact with each other may change from the appropriate positions.